Science: Tiny wire could make molecular electronics a reality

Chemists in Australia have designed and synthesised a molecule that
could be used as a ‘molecular wire’. Such a wire might link tiny electronic
devices, making molecular-scale electronics a reality.

Max Crossley and Paul Burn of the University of Sydney joined together
four so-called porphyrin units to make a conducting strand, 6.5 nanometres
long (Journal of the Chemical Society, Chemical Communications, 1991, p
1569).

Porphyrin molecules are common in nature, where they are generally involved
in biochemical processes that require the transfer of electrons. An example
is chlorophyll in green plants, which is a porphyrin-based pigment. Porphyrins
are also at the heart of haemoglobin, the molecule that carries oxygen in
the blood, and vitamin B12.

Crossley says that it was seeing porphyrins in so many different but
loosely related natural systems that gave him and his colleagues the idea
of joining them together to make a molecular wire.

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A molecular wire must possess several properties to be of any use, say
the researchers. First, it must be able to conduct electrons along its length.
Crossley and his colleagues achieved this by making a wire in which each
porphyrin in the chain possesses a system of so-called pi-electrons. These
form alternating double bonds between the carbon and nitrogen atoms in the
rings, and it is the movement of this ‘conjugated’ double-bond system that
carries the current. The chemists joined the porphyrins to each other with
benzene rings.

If the pi-electrons are to move easily along the wire, the porphyrins
and benzene rings must all lie in the same plane (see diagram). The chemists
demonstrated that this was so by building a molecular model of the system.

Crossley believes that by adding metal ions to the system, it should
be possible to ‘fine tune’ the energy required to make a current flow. By
bonding to the centre of the porphyrin rings, such ions would alter the
energy levels of the conducting and nonconducting states of the wire. More
than 50 metals can bond to porphyrins, says Crossley, so it should be easy
to find one with the optimal energy gap.

For a molecular wire to be viable it must also possess functional chemical
groups which can be oxidised easily (lose electrons) or reduced easily (gain
electrons). Only if this is the case, can a current be initiated at one
end of the wire and tapped at the other.

Crossley and his colleagues again took their lead from nature. Porphyrin
systems are happy to be involved in ‘red-ox’ reactions. If the right functional
groups are added to the ends of the wire, it might be possible to control
the terminals – by shining light on them, for instance. The ends would become
molecular switches.

A third feature that a molecular wire must have is an insulating sheath
to prevent the current leaking to the surroundings. To achieve this, the
chemists attached bulky organic tert-butyl groups to the backbone of the
molecule. The useful side-effect of incorporating such a sheath into the
wire is that the molecule becomes quite soluble in organic solvents, such
as chlorohydro-carbons. This should make it easier to manipulate, and to
turn into circuits.

The final property that a molecular wire must have is a defined and
fixed length. It must also be capable of spanning the gap between support
structures, which in prac-tice might be lipid membranes. The dimensions
are largely determined by the width of a porphyrin ring, but because the
chemists’ synthesis is quite general, they have been able to make a range
of wires between 5 and 12 nanometres long.

Crossley and his team are now working on what they call molecular ‘alligator
clips’. As the name implies, these would join the ends of the wire to the
support material. The chemists have also developed a wire with a 90 degrees
bend in it. They say it will be quite useful for connecting devices round
corners.

Jean-Marie Lehn of the Institut le Bel at the Louis Pasteur University,
Strasbourg, says: ‘Crossley’s wire is a very exciting development.’ He adds
that: ‘Such rigid molecular wires will hopefully allow chemists to join
functional groups in a controlled and organised manner.’

Lehn and his colleagues are working on molecular wires of a different
sort. They have designed a charged molecule, called caroviologen, which
resembles a naturally occurring compound called carotene. The researchers
have used this molecule to mediate electron transfer across a bilayer membrane.
The membrane, made of spherical globules known as vesicles, is reducing
(electron giving) on the outside and oxidising (electron withdrawing) on
the inside.

Lehn believes that it might be possible to make a ‘molecular rectifier’
with the caroviologen wire and the right functional groups. A diode rectifies
an alternating current, and will be vital in molecular elec-tronics. (Journal
of the Chemical Society, Chemical Communications, 1991, p 1179).